It is known that the braking system of a vehicle is designed for the case of maximum load. If, in this case, it is assumed that the vehicle is in a braking range where the anti-locking device does not have to be activated, a pressure medium is applied to all brake cylinders in proportion to the opening of the brake valve. The applied pressure in this case is equal to the pump pressure. This causes the forces applied to the front and rear axles of the vehicle to be equal.
In the case of an unloaded vehicle, the following takes place: since the load of the vehicle acts, as a rule, on the rear axle, the braking force acting on the front axle is, in this case, proportionally greater than that acting on the rear axle. Since the brake system is designed primarily for a fully loaded vehicle, and not usually for an empty vehicle, this means in the case under discussion that, as a result of the lack of load on the rear axle, the rear wheels are locked first when the brake is applied and then the front wheels. One of the requirements of brake design is, however, that the front wheels should be locked first and subsequently the rear wheels.
In order to prevent the rear wheels in an empty or partially loaded vehicle from being locked before the front wheels, it is therefore necessary at a certain point to interrupt or restrict the flow of pressure medium to the brake cylinders of the rear wheels. Such control devices are known. These control devices cause the force acting on the front axle of the vehicle to increase in proportion to the applied pressure, while the force acting on the rear axle remains constant or increases at a proportionally slower rate after passing a point corresponding to the load, hereinafter called load-changing points, independently of the applied pressure.
The known braking force control devices are preadjusted by means of an adjusting lever. A tension spring which is acted on by a system of levers adjustable as a function of the load, is connected to the adjusting lever.
The system or device is designed in such a way that a linear relationship exists between the braking pressures required for corresponding to load variations, and the force acting on the adjusting lever or the deflection of the tension spring, which may be expressed by the equation p=c.times.s wherein p is braking pressure, c is vehicle load, and s is spring deflection. Vehicles are frequently equipped, for example with leaf springs. Due to the usually progressive deflection characteristic of these leaf springs, the amount of deflection of the spring and the average rate thereof is greater for vehicle loads within a given range of low loads, than the smaller amount and rate of deflection of the spring in a given range of higher loads. This progressive deflection character of the leaf spring system of the vehicle has an effect on the above-mentioned linear relation of pressure and spring deflection.
The law requires a certain so-called braking range, f, so that the function p=f(s) must be chosen in accordance with this range. As a rule, this range requires a progressive character, so that there is a danger that the function p=f(s) falls outside this range for certain load conditions unless additional steps are taken. Known solutions are special lever designs, for instance designs which cause the displacement of the lever to be transferred progressively to the tension spring.
The known load responsive control device systems exhibit as a whole the following disadvantages. Due to the fact that the movement of the lever is used to more or less pretension the control spring, relatively large forces act on the lever system and this fact requires that the lever system must be built to be correspondingly stable, heavy and voluminous. These large forces must be transmitted, which increases the cost of the structure. These known control devices are therefore force-operated.
The danger consists of the fact that fatigue phenomena will occur in the pretensioned tension spring. During operation of the vehicle, either excessive or insufficient movements will occur in the lever-system transmitting the forces, for example as the result of potholes. This creates the danger of overloading, which may damage the spring. In order to prevent such overloading, a more expensive design is required.
An important disadvantage of the known control devices is that forces act on the entire load-dependent adjusting system even when the brakes are in a release state, for example, because of axle oscillations; this creates the danger of premature wear.
Even if the above-mentioned danger of overloading caused by excessive movements can be largely eliminated by providing suitable cam surfaces in the adjusting device, the disadvantage still persists that oscillations of the axles are transmitted and that forces are still applied even in the brake-release condition.